Cutis laxa is a connective tissue disease of the skin of unknown cause, characterized by sagging skin, premature wrinkling and reduced skin elasticity. It occurs in heritable and acquired forms. Although classically the disease only affects the skin it appears to occur in many variations or subsets affecting a variety of other organs in some patients. An animal model to study the molecular mechanisms of cutis laxa would be extremely useful. Recently, the existence of mice carrying the Sagg mutation was described (1). These mice were named Sagg because they have heritable saggy or loose skin that resembles characteristics of skin found in patients with cutis laxa. Associated with the loose skin is a profound reduction of collagen and elastin in the dermis. We propose this loose skin mutant mouse strain, named Sagg, as a model for the heritable form of cutis laxa. In this R21 application, we intend to refine the position of the Sagg mutation and to characterize the phenotype of this mutant mouse to determine whether it is a suitable model for cutis laxa. In Specific Alm I we will obtain Sagg N2 mice from an intersubspecific backcross of (1231-1/140Sagg/+ x CAST/ei)Fl x CAST/ci and construct a fine-map of the recombination events between the molecular markers, DIMU232 and DIAM234. We will delimit the region in which Sagg resides to less than 2 cM, as a preliminary but necessary step for chromosome walking experiments aimed at locating and cloning the gene. In Specific Aim 2, we will characterize the phenotype of Saggl+ mice at monthly intervals from ages I to 12 months by performing histological and biochemical analyses on the skin, lungs, kidney, heart and aorta. These studies will include in situ hybridizations to determine the spatial and temporal expression of collagen types I and III and elastin mRNAs in these tissues. Dermal fibroblasts will be used for Northern blot analyses to examine the steady state mRNA levels of additional genes which will include: A; candidate genes which have been identified from the results of Specific Aim 1, and B; genes encoding extracellular matrix components (EMC) which might be expected to be affected by the Sagg mutation. These experiments will also include genes encoding enzymes involved in the degradation and removal of the EMC. These studies will be undertaken in order to elucidate the downstream molecular events resulting from the Sagg mutation.